Stability of hydrocarbons containing asphal tenes

ABSTRACT

Heavy fuel oils or residual fuel oils can be stabilized with magnesium over-based compounds such as magnesium overbased carboxylates. It was surprisingly discovered that adding magnesium overbased carboxylates to the residual fuel oils shortly after thermal cracking gave much better results than can be achieved after the application of the carboxylates to the fuel oil after storage. Further, compounds containing at least about 21 wt % magnesium also give better results than compounds with 18 wt % or less, in one non-limiting embodiment. Magnesium overbased compounds can also be added to coker feedstocks to reduce coker furnace fouling. Treatment with the methods of this invention reduces asphaltene deposits and sludges.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 60/488,891 filed Jul. 21, 2003.

FIELD OF THE INVENTION

The present invention relates to methods and compositions to stabilizehydrocarbon streams containing asphaltenes, and more particularlyrelates, in one embodiment, to methods and compositions to stabilizeresidual fuel oils and coker feedstocks using readily availablematerials.

BACKGROUND OF THE INVENTION

The stability of heavy fuel oils obtained from thermally crackedresidual oils is a well known problem with significant economicramifications. Residual fuel oil consists predominantly of an oil phase,the composition of which is almost entirely related to the crude oilfrom which it originates. In this oil phase are dispersed relativelylarge hydrocarbon molecules called asphaltenes. It is the nature ofasphaltenes to be attracted to one another, and it is this tendency,along with size and concentration of the asphaltene molecules, that areconsequences of both the crude oil type and the thermal crackingmanufacturing process. The compositions of the various thermally crackedresidual fuel oils can thus vary widely.

The stability of a residual fuel oil can be defined as its ability toresist the formation of carbonaceous sludge during storage and handling.The effects of sludge formation in a residual fuel oil in systems wherethat fuel oil is used to power an engine can result in chokedcentrifuges, filter blocking, heater fouling, and ultimately, engineshut down and damage. However, the simple formation of sediment overtime in the bottom of storage tanks causes problems because these sludgelayers are difficult to remove. These sediments are due to theaggregation of the unstable, high molecular weight polynuclear aromaticasphaltenes.

The traditional approaches of trying to stabilize thermally cracked fueloils is to blend them with valuable refinery stocks or add any one of avariety of different chemicals to the fuel oils stored in tanks.However, these techniques have the disadvantage of having to becustomized for each particular fuel oil. Moreover blending of fuel oilwith other refinery cutter stocks requires the availability of aromaticheavy boiling cuts from Fluid Catalytic Cracking plants. If such streamsare not available, any attempt to blend unstable cracked fuel withatmospheric or vacuum gas-oil will result in a de-stabilization ofasphaltenes. Addition of chemicals in storage tanks also requires goodmixing, which is seldom available.

Similar stability problems affect visbreaking and delayed cokingprocesses, and potentially any bottoms upgrading process where the feedis stored at elevated temperatures prior to processing. Although delayedcoking is used herein as a specific embodiment, as delayed cokers areunits where the problem is often seen, it will be appreciated that theproblem is present in any operation where feed is preheated and heatexchangers experience fouling.

Delayed coking is a bottoms upgrading process. It involves raising afeedstock to approximately 950° F. (510° C.) using a process furnace,and then transferring the hot stream to a coke drum. The coke drumfunctions as a residence chamber for the oil to allow time for crackingto occur. The products of the cracking are coke (a highly enrichedcarbon polymer) which forms in the coke drum, and some quantity ofcracked distillates (gasoline and gas oil boiling range) which areremoved by fractionating a stream that leaves the coke drum. A commonproblem with this process is the formation of fouling (coke formation)in the process furnace.

Delayed coker furnace fouling is believed to result from at least twomechanisms. The first involves the pyrolysis of hydrocarbons, followedby polymerization and dehydrogenation, leaving behind a nearly purecarbon structure commonly called coke. The second mechanism involves thedestabilization of already existing asphaltene polymers in thefeedstock. These asphaltenes exist as a colloidal dispersion in thefeedstock, with another class of high boiling hydrocarbons, resins,acting as the dispersing agent. Any of several changes to which thefeedstock is exposed can disturb this colloidal state, withprecipitation of the asphaltenes on the furnace tubes. These asphaltenesfurther dehydrogenate and result in a coke-like residue, very similar tothat derived from the former mechanism.

The feed to the coker unit is typically composed of crude unit vacuumtower residue or bottoms (VTB). In most coker units, the VTB is partlyrouted directly as coker feed, while a portion is routed to intermediatestorage. The storage exists as a buffer, to allow the upstream crudeunit to continue producing VTB even while the coker unit is down forfurnace tube decoking. This decoking is periodically necessary to removethe coke formed from the two mechanisms described above. The primaryeconomic impact of this furnace coking or fouling includes lostproduction penalties and potentially shorter furnace tube life.

There is thus a need to find a method and/or composition that will helpstabilize thermally cracked fuel oils and coker feedstocks that is moreeffective than current techniques.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide achemical composition for improving the stability of thermally crackedresidual fuel oils.

It is another object of the present invention to provide a method fortreating thermally cracked residual fuel oils that improves theirstability.

An additional object of the invention is to provide a fuel oil that hasimproved stability.

In carrying out these and other objects of the invention, there isprovided, in one form, a method for stabilizing a hydrocarbon streamcontaining asphaltenes that involves heating the hydrocarbon streamcontaining asphaltenes; and adding to the hydrocarbon stream a magnesiumoverbased compound. The magnesium overbased compound may be a magnesiumoverbased carboxylate, a magnesium overbased sulfonate, a magnesiumoverbased phenates, or mixtures thereof. The magnesium overbasedcompound is added in an amount effective to improve the stability of thehydrocarbon stream. The hydrocarbon stream is subjected to heat afterthe addition.

Further provided in another non-limiting embodiment is a method forinhibiting coke furnace fouling that includes heating a coke drumfeedstock containing asphaltenes and then adding to the coke drumfeedstock a magnesium overbased compound. The magnesium overbasedcompound can be a magnesium overbased carboxylate, a magnesium overbasedsulfonate, a magnesium overbased phenate, or mixtures thereof. The cokedrum feedstock is then stored at an elevated temperature.

There is additionally provided a method for stabilizing heavy fuel oilsthat involves thermally cracking a residual oil to provide heavy fueloil; and adding to the thermally cracked heavy fuel oil a magnesiumoverbased compound that is a magnesium overbased carboxylate, amagnesium overbased sulfonate, and/or a magnesium overbased phenate, inan amount effective to improve the stability of the fuel oil, where theadding is conducted sufficiently soon after thermal cracking to improvestability. In one non-limiting embodiment of the invention, themagnesium overbased carboxylate is added within about 2 hours or less ofthermally cracking the fuel oil. In another non-limiting embodiment ofthe invention, the magnesium overbased carboxylate is added within about40 hours or less of thermally cracking the fuel oil.

There is additionally provided in another non-restrictive form of theinvention a stabilized heavy fuel oil that includes a thermally crackedresidual oil. The stabilized heavy fuel oil also includes a magnesiumoverbased compound in an amount effective to improve the stability ofthe fuel oil. The magnesium overbased compound is added to the thermallycracked residual oil sufficiently soon after thermal cracking to producethe residual oil to improve stability. The magnesium overbased compoundmay be a magnesium overbased carboxylate, a magnesium overbasedsulfonate, and/or a magnesium overbased phenate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the addition of overbased magnesiumcompounds relatively soon after the heavy fuel oil is produced bythermal cracking before it is stored and/or blended, if blending isnecessary. In particular, the invention is concerned with the treatmentof visbreaker tars. According to one embodiment of the invention, theapplication of these compounds is much more effective if they areapplied to the visbreaker tars before storage and blending of the tar.In particular, early application of these compounds helps prevent orreduce aging difficulties, resulting in far better results than attemptsto cure aging and stability problems after their occurrence.

In one non-limiting embodiment of the invention, the method operates bytreating heavy fuel oils from fractionation of thermally crackedatmospheric or vacuum residuals. By “heavy” is meant with a boilingrange above 350° C., a density ranging from 0.9 to 1 kg/m³ and aviscosity range from 200 to 500 centistokes at 50° C. These propertiesare averages reported as non-limiting examples only; it should be clearthat these parameters are not comprehensive of all thermally crackedresiduals to which the present invention applies. The exact method ofproduction of the fuel oils and their characteristics are not criticalto the method of this invention. Producing heavy fuel oils by thermalcracking of residual oil is a well-known process in the industry. Withinthe context of this invention, it will be understood that the term“heavy fuel oil” includes, but is not necessarily limited to, visbreakertars (or vistars), thermally cracked resids (residual fuel oils),turbine fuels, and the like.

Suitable overbased magnesium compounds include, but are not necessarilylimited to, magnesium overbased carboxylates, magnesium overbasedsulfonates and/or magnesium overbased phenates and the like. In onenon-limiting embodiment, magnesium overbased carboxylates may be used. Aparticularly suitable magnesium overbased carboxylate is KI-85 availablefrom Baker Petrolite. In another non-limiting embodiment of theinvention, the overbased magnesium compound contains from about 21 toabout 26 wt % magnesium. In another non-limiting embodiment of theinvention, the magnesium content is from about 24 to about 25 wt %. In aparticular non-limiting embodiment of the invention, the compound has atleast about 21 wt % magnesium, and in another non-limiting embodiment,has at least about 25 wt % Mg. Typically, these magnesium proportionsare average amounts. These magnesium overbased compounds may be readilyproduced by methods well known in the art.

In another non-limiting embodiment of the invention, the residual fueloil is treated by the addition of about 25 to about 2000 ppm of asuitable magnesium overbased compound, based on the heavy fuel oil. Inanother non-limiting embodiment, the proportion of the magnesiumoverbased compound ranges from about 150 to about 2000 ppm of based onthe heavy fuel oil.

An important part of the method of the invention is to add the magnesiumoverbased carboxylate (or other compound) to the thermally cracked heavyfuel oil sufficiently soon after thermal cracking to improve itsstability. The optimal time or time range of addition will varydepending on the nature of the overbased magnesium compound, how much isadded, how much magnesium is present in the compound, the temperature ofaddition and the nature of the thermally cracked fuel oil. In onenon-limiting embodiment, the overbased magnesium compound is added atleast within about 40 hours or less. In another non-limiting embodimentthe overbased magnesium compound is added at least within about 2 hoursor less after separation of heavy fuel oil from other thermally crackedstreams. In another non-limiting embodiment of the invention, themagnesium overbased compound is added within 20 hours or less afterthermal cracking, and in an alternate embodiment, within 10 hours orless.

In one other non-limiting embodiment of the invention, the adding of theoverbased magnesium compound is performed within a temperature range ofabout 250 to about 490° C. Typically, this temperature will be at ornear the temperature of the thermally cracked residual fuel oil shortlyafter it is produced. In yet another non-limiting embodiment, theoverbased magnesium compound is added within a temperature range ofabout 250 to about 380° C.

Although it is acceptable to use the overbased magnesium compounds madeaccording to the methods of U.S. Pat. No. 6,197,075, incorporated hereinby reference, it will be appreciated that in another non-limitingembodiment of the present invention, the method is practiced in theabsence of adding a copromoter reaction product of a succinic anhydrideand a lower carboxylic acid, in all of the forms described in the '075patent. It is also noted that the method of the '075 patent does notappreciate the need for adding the overbased magnesium compounds veryshortly after the residual fuel oils are produced by thermal cracking.

Further, it will be appreciated that it is not necessary for the heavyfuel oils to completely prevent asphaltene sediments or aggregation orto produce a heavy fuel oil that is stable forever for the invention tobe considered successful. Rather, the methods and compositions of thisinvention are successful if the stability of the heavy fuel oils issimply improved.

The present invention also relates to inhibiting or preventing furnacefouling caused by asphaltenes in other hydrocarbon streams includingcoker feedstocks. Again, it will be appreciated that fouling need not beentirely prevented for the invention to be considered successful in thiscontext.

It has been observed that furnace fouling rates are correlated to theamount of coker unit feed that comes from storage. Higher amounts offeed from storage result in generally higher rates of furnace fouling.The basis for the invention is the possibility that a significantportion of the coker furnace fouling results due to the 10-20% of feedthat comes from the storage tank. This stored-feed-stock-induced foulingis believed to result from degradation of the feedstock during thisstorage time. These materials are very viscous, and, as such, must bekept at elevated temperatures (250° F. and higher; 121° C. and higher)while stored to allow pumping to the coker unit. Storage times can rangefrom a few days to several weeks.

The degradation of the stored materials is believed to involve severalpossible mechanisms:

-   -   1. Thermal destruction of the resins, followed by aggregation of        asphaltenes.    -   2. Oxidative destruction of the resins, followed by aggregation        of asphaltenes.    -   3. Oxidation and polymerization of hydrocarbons to form        additional, poorly soluble polymers.

In all three cases, metal catalysis may promote the chemical reactions.The metal catalysis may be caused by impurities in the hydrocarbonstream or possibly the metal conduits and vessels. Also, in cases 1 and2, the stability of asphaltenes is negatively affected, which causesthis portion of the coker feed to exhibit a higher degree of asphaltenedestabilization and precipitation than the portion of feed that does notexperience extended storage time. In case 3, a polymer is formed that,like asphaltenes, has poor solubility in the oil, and with the extremetemperatures seen in the furnace, can precipitate as foulant.

Finally, it has been discovered that these forms of degradation can besuccessfully inhibited using the magnesium overbased compoundspreviously described. A variety of thermally stable dispersants such asthe magnesium overbased compounds have shown promise in controlling thisdegradation, thus greatly reducing this component of the furnacefouling. Laboratory testing thus far has shown that two magnesiumoverbased compounds within the definition of this invention, boththermally stable dispersants, give significant inhibition to thedegradation, as measured by solids by hot filtration.

Previous attempts to control or reduce furnace fouling in delayed cokingunits and visbreakers have generally met with little success. Onlysporadic success has been reported. The prior attempts have involvedadding either a dispersant, anti-coking or antioxidant additive directlyto the feed that goes to the furnace in the delayed coker unit.Additionally, previous attempts to improve on the furnace treatment bymoving the injection point back up stream, as far as the vacuum towerbottom, to enhance mixing of additive and oil and to provide additionalresidence time appear to have shown some benefit. The benefit may wellhave occurred as a result of some of the additive going with the feedthat was stored. This invention involves introducing a more concentratedchemical treatment into the stored feed or just prior to the hydrocarbonstream being stored. By “more concentrated” is meant the high-magnesiumcontent compounds of this invention.

Typically in a delayed coking operation, the stored feed comes from apair of storage tanks. One embodiment of this invention would involveconcentrating the treatment chemicals in the rundown to storage, ratherthan treating the furnace directly.

The invention will now be further described with respect to particularExamples that are not meant to limit the invention, but rather areintended to illustrate it further with respect to certain, more specificnon-limiting embodiments.

Hot Filtration Test

The Hot Filtration Test (HFT) is a relatively standard test to determinethe stability of a particular fuel oil.

Materials

-   -   Four Whatman fiber glass GF/A type filters, 1.6 micrometers        porosity or equivalent    -   Hot filtration test equipment    -   Heating plate    -   Analytical balance, 0.0001 grams    -   100° C. thermometer with 1° C. precision    -   n-Heptane, analytical grade    -   Mixture of 85% n-heptane, 15% xylene by volume, analytical grade        Procedure:

Install on a hot filtration test apparatus four pre-weighed (0.0001grams precision) filters, two for each filtration heated flask. Heat thefiltration flasks at 100° C. with vacuum applied to the hot filtrationtest filter holders.

Heat the fuel to about 70-80° C. to have a fluid fuel. Weigh 10 grams ofheavy fuel oil sample, with 0.0001 grams precision. Heat fuel oil in arange of 99-101° C. and pour about half of the fuel oil in the first HFTfilter holder, with the two Whatman filters. Register, by weight, theexact quantity poured. Repeat the operation with the other filterholder. Wait for complete filtration under vacuum. Cool filter holdersto ambient temperature. Wash each filter couple, with filters on thefilter holder, with two washings of 25 mls each of n-heptane/xylenemixture and two washing of 10 mls each of n-heptane. Dry each filtercouple, and reweigh, with 0.0001 gram precision.

Sediments are determined by average of the weight difference after andbefore filtration. Hot filtration results can be calculated as follows.HF1=First couple of filters weight after HFT test procedure−First coupleof filters, 1 weighed after HFT, 1 before HFTHFT1=100*(HF1)/Fuel oil filteredHF2=Second couple of filters 1 weight after HFT test procedure−Secondcouple of filters, 1 weighed after HFT, 1 before HFTHFT2=100*(HF2)/Fuel oil filteredHot Filtration Test Result, HFT=(HFT1+HFT2)/2Results

As widely recognized, the acceptable sediment content for fuel oil isless than 0.5% by hot filtration test (HFT). These sediments are due toaggregation of unstable high molecular weight polynuclear type aromaticsknown as asphaltenes. Higher contents than 0.5% have a negative impacton fuel filters (plugging) and on the burning quality of fuel. Highcontents of sediments also have a negative impact on storage tanks asthey tend to settle out on the bottom of the tank with a layer of sludgethat is difficult to remove.

The invention consists in limiting the content of sediment formationwith time in storage tanks for fuel oil. Particularly, the invention isconcerned with treatment of residues from thermal cracking, used asheavy fuels or blended with gas-oils for fuel oil no. 6 production, inone non-limiting embodiment. More particularly, this invention isrelated to the treatment of resids from visbreaking, commonly known asvistar or tar.

These feedstocks are very problematic with respect to sediment formationsince thermal cracking in the furnace gives rise to instability. Thiscan be partially solved by decreasing thermal cracking temperatures orreaction time at cracking temperatures (about 430-490° C.), althoughthis leads to a strong decrease in the yield of valuable 360°C.+distillates from thermal cracking.

In attempts to limit the problem of sludge formation, several productswere tested: oil soluble magnesium carboxylate overbased products(having 14-18% magnesium), oil soluble magnesium carboxylate overbasedformulations with a higher magnesium content (23-26%; average 25%), anasphaltene dispersant (Baker Petrolite BPR34260) and a stericallyhindered phenol, which acts as a radical stopper-scavenger, commonlymarketed as antioxidant.

To test the products' effectiveness, samples of vistar and vistarblended with gas-oil streams were submitted to meet no. 6 fuel oilviscosity specifications, the blank (untreated) samples and treatedsamples were subjected to controlled “aging”, that is, keeping them at80° C. for a period of time of more than 100 hours. This is wellrepresentative of typical storage tank temperatures and after more than60-80 hours aging (sludge/aggregates) is complete.

Products are qualitatively considered to be effective whenever they areable to keep a sediment content of less than 0.5% by Hot FiltrationTest.

Reported results are from duplicated measurements with differences inmeasurements of less than 10%.

The results on samples from storage tanks, immediately after storage,show an unexpected ineffectiveness of these products. The charge fromwhich the tar sample came had been processed in a visbreaker plant about10 hours before the sampling of finished product, the tar. In fact, theinitial Hot Filtration Test value of the tar is low (0.07%), andproducts are ineffective even at higher dosages as reported in Table Ibelow. TABLE I HFT Data for Samples Partially Aged, From Storage Sample,after Dosage, storage (10 hours Aged, Ex. Product ppm after production)108 hours 1 None 0 0.07 1.25% 2 Overbased 24-26% Mg 2000 0.93% 3Overbased 23-26% Mg 200 0.95% 4 Overbased 14-18% Mg 2000 1.01% 5Dispersant BPR34260 5000 0.01 1.25% 6 Sterically hindered phenol 50000.01 1.23% antioxidant BPR34017BPR34260 and BPR34017 are available from Baker Petrolite.

It was discovered with several trials that for thermally cracked resids(tar), magnesium carboxylates products are surprisingly effective whenadded immediately (within about 2 hours or less) after separation of tarfrom other thermal cracking (visbreaking) products, before sending tarto storage in tanks, as shown below in Table II. TABLE II HFT Data forFresh Samples from Plant, Treated Immediately After Tar FractionationDosage, Fresh Aged, Ex. Product ppm Sample 108 hrs 7 None 0 0.01 1.21% 8Overbased 25% Mg 2000 0.01 0.31% 9 Overbased 25% Mg 200 0.01 0.38% 10Overbased 25% Mg 150 0.01 0.38% 11 Overbased 25% Mg 100 0.01 0.71% 12Overbased 25% Mg 50 0.01 0.79% 13 Overbased 14-18% Mg 2000 0.01  0.7% 14Overbased 14-18% Mg 200 0.01 0.95% 15 Dispersant BPR34260 2000 0.01 1.0% 16 Sterically hindered phenol antioxidant 2000 0.01 1.05% BPR34017

It should be noted that magnesium carboxylate with a 25% averagemagnesium content was effective to keep asphaltenes aggregationresulting in Hot Filtration Test sediments below 0.5% with dosages of2000 ppm, 200 ppm, and 150 ppm (Examples 8, 9 and 10, respectively). Forthe magnesium over-based based products of these Examples, 25% is anaverage value, as this product has some variability in the exactmagnesium content, typically between 23 and 26%.

It was also discovered that treatment on fuel oil no. 6 from blending ofgas oil and tar and tar itself is ineffective to reduce Hot FiltrationTest content with samples from storage tanks that have been aged formore than 60 hours and are beyond the 0.5% HFT content limit, as shownin Table III. TABLE III HFT Data on Samples from Storage Tank after 60hrs for Tar and for Blended Tar to Meet No. 6 Fuel Oil ViscositySpecifications Sample, from Ex. Product Dosage, ppm storage tank 17 None0 >0.5% 18 Overbased, 24-26% Mg 2000 >0.5% 19 Overbased, 24-26% Mg200 >0.5% 20 Overbased, 14-18% Mg 2000 >0.5% 21 Overbased, 14-18% Mg 200>0.5%Generally, blended tars (fuel oil no. 6) showed a greater HFT thanunblended tars.

Example 22 Prevention of Furnace Fouling

A commercial coker currently takes approximately 15% of its feed fromstorage. In a 5 year period, numerous additive treatments have beentried, at the furnace, to control furnace fouling. The severity of theproblem is such that a spalling (a cleanup process) is necessary, onaverage, every 12 days. Furnace fouling is measured by the rate of skintemperature change for several thermo-couples attached to furnace tubes.A typical starting temperature is 1000° F. (538° F.). The limitation isan upper limit on the skin temperature, typically around 1200° F. (649°C.). When the limit is reached, a spall or a decoke operation to removecoke is required. When either cleanup process is carried out, productionis lost, and furnace tube life is shortened. These costs are what drivethe refiner to seek solutions. It is expected that injection of aneffective amount of a magnesium overbased carboxylate, such as theproportions previously mentioned, would inhibit fouling sufficiently thetime between cleanings is increased from 12 days to 3 months.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing a method of improving the stability of heavy fueloils and other hydrocarbon streams containing asphaltenes. However, itwill be evident that various modifications and changes can be made tothe inventive compositions and methods without departing from thebroader spirit or scope of the invention as set forth in the appendedclaims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, particularmagnesium-containing overbased compounds falling within the claimedparameters and added at different times and dosages, or with particularco-components, but not specifically identified or tried in a particularcomposition or under specific conditions, are anticipated to be withinthe scope of this invention.

1. A method for stabilizing a hydrocarbon stream containing asphaltenescomprising: heating the hydrocarbon stream containing asphaltenes; andadding to the hydrocarbon stream a magnesium overbased compound selectedfrom the group consisting of magnesium overbased carboxylates, magnesiumoverbased sulfonates, magnesium overbased phenates, and mixturesthereof, in an amount effective to improve the stability of thehydrocarbon stream, where the hydrocarbon stream is subjected to heatafter the addition.
 2. The method of claim 1 where the magnesiumoverbased compound is added in an amount ranging from about 25 to about2000 ppm based on the hydrocarbon stream.
 3. The method of claim 1 wherethe magnesium overbased compound contains at least 21 wt % magnesium. 4.The method of claim 1 where the adding is performed within 40 hours orless after the hydrocarbon stream is thermally cracked.
 5. The method ofclaim 1 where the hydrocarbon stream is a coke drum feedstock and themagnesium overbased compound is added to the feedstock prior to storingthe feedstock at an elevated temperature.
 6. The method of claim 1 wherethe adding is performed within a temperature range of about 250 to about490° C.
 7. A method for inhibiting coke furnace fouling comprising:heating a coke drum feedstock containing asphaltenes; adding to the cokedrum feedstock a magnesium overbased compound selected from the groupconsisting of magnesium overbased carboxylates, magnesium overbasedsulfonates, magnesium overbased phenates, and mixtures thereof; andstoring the coke drum feedstock at an elevated temperature.
 8. Themethod of claim 7 where the magnesium overbased compound is added in anamount ranging from about 25 to about 2000 ppm based on the coke drumfeedstock.
 9. The method of claim 7 where the magnesium overbasedcompound contains at least 21 wt % magnesium.
 10. The method of claim 7where the adding is performed within a temperature range of about 250 toabout 490° C.
 11. A method for stabilizing heavy fuel oils comprising:thermally cracking a residual oil to provide a heavy fuel oil; andadding to the heavy fuel oil a magnesium overbased compound selectedfrom the group consisting of magnesium overbased carboxylates, magnesiumoverbased sulfonates, magnesium overbased phenates, and mixturesthereof, in an amount effective to improve the stability of the fueloil, where the adding is conducted sufficiently soon after thermalcracking to improve stability.
 12. The method of claim 11 where themagnesium overbased compound is added in an amount ranging from about 25to about 2000 ppm based on the heavy fuel oil.
 13. The method of claim11 where the magnesium overbased compound contains at least 21 wt %magnesium.
 14. The method of claim 11 where the adding is performedwithin 40 hours or less of the thermal cracking.
 15. The method of claim11 where the adding is performed within a temperature range of about 250to about 490° C.
 16. The method of claim 11 where the method ispracticed in the absence of adding a co-promoter reaction product from asuccinic anhydride and a lower carboxylic acid.
 17. The method of claim11 where the magnesium overbased compound is a magnesium overbasedcarboxylate.
 18. A method for stabilizing heavy fuel oils comprising:thermally cracking a residual oil to provide a heavy fuel oil; andadding to the heavy fuel oil a magnesium overbased carboxylate in anamount effective to improve the stability of the fuel oil, where theadding is conducted sufficiently soon after thermal cracking to improvestability, where the magnesium overbased carboxylate contains at least21 wt % magnesium and the adding is performed within 2 hours or less ofthe thermal cracking.
 19. The method of claim 18 where the magnesiumoverbased carboxylate is added in an amount ranging from about 25 toabout 2000 ppm based on the heavy fuel oil.
 20. The method of claim 18where the adding is performed within a temperature range of about 250 toabout 490° C.
 21. The method of claim 18 where the method is practicedin the absence of adding a co-promoter reaction product from a succinicanhydride and a lower carboxylic acid.
 22. A stabilized heavy fuel oilcomprising: a heavy fuel oil prepared by thermally cracking a residualoil; and a magnesium overbased compound in an amount effective toimprove the stability of the fuel oil, where the magnesium overbasedcompound is added to the heavy fuel oil sufficiently soon after thermalcracking to produce the residual oil to improve stability, and where themagnesium overbased compound is selected from the group consisting ofmagnesium overbased carboxylates, magnesium overbased sulfonates,magnesium overbased phenates and mixtures thereof.
 23. The heavy fueloil of claim 22 where the magnesium overbased compound is present in anamount ranging from about 25 to about 2000 ppm based on the heavy fueloil.
 24. The heavy fuel oil of claim 22 where the magnesium overbasedcompound contains at least 21 wt % magnesium.
 25. The heavy fuel oil ofclaim 22 where the magnesium overbased compound is added within 40 hoursor less of the thermal cracking.
 26. The heavy fuel oil of claim 22where the magnesium overbased compound is added within a temperaturerange of about 250 to about 490° C.
 27. The heavy fuel oil of claim 22further comprises an absence of a copromoter reaction product from asuccinic anhydride and a lower carboxylic acid.
 28. The heavy fuel oilof claim 22 where the magnesium overbased compound is a magnesiumoverbased carboxylate.
 29. A stabilized heavy fuel oil comprising: aheavy fuel oil prepared by thermally cracking a residual oil; and amagnesium overbased carboxylate in an amount effective to improve thestability of the fuel oil, where the magnesium overbased carboxylate isadded to the heavy fuel oil within 2 hours after thermal cracking toproduce the residual oil to improve stability, and where the magnesiumoverbased carboxylate contains at least 21 wt % magnesium.
 30. The heavyfuel oil of claim 29 where the magnesium overbased carboxylate ispresent in an amount ranging from about 25 to about 2000 ppm based onthe heavy fuel oil.
 31. The heavy fuel oil of claim 29 where themagnesium overbased compound is added within a temperature range ofabout 250 to about 490° C.
 32. The heavy fuel oil of claim 29 furthercomprises an absence of a copromoter reaction product from a succinicanhydride and a lower carboxylic acid.